The invention relates to a welding system having a monitoring device.
Normally, in industrial manufacturing, welding operations are carried out semi-automatically or fully automatically by means corresponding automatic welding machines. The welding operation itself can be carried out by means of different welding tools, for example, welding torches or by means of lasers. In each case, it is desirable to control the welding result.
During the control of welding operations, particularly when point welding and stud welding, it has been attempted to carry out a monitoring on the basis of ultrasound. This leads to results which so far have not been satisfactory. Other measuring devices, such as laser vibrometers or similar devices, on the one hand, require high expenditures and, on the other hand, are very sensitive with respect to environmental influences, such as smoke, dust, sparks, etc.
In many welding processes, for example, during resistance welding or arc welding, the monitoring of the welding quality is therefore advantageous.
For this reason, welding systems already exist which have a monitoring device (UMEAGUKWU, C. et al.; Robotic Acoustic Seam Tracking: System Development and Application; IEEE Transactions on Industrial Electronics; Vol. 36; No. 3; P. 338-348; 1989). However, these require comparatively high expenditures and are relatively sensitive to outside environmental influences.
It is therefore an object of the invention to suggest a system by means of which a welding operation can be monitored without any high expenditures and without any contact, which welding operation is insensitive to outside environmental influences.
This object is achieved by a welding system having a monitoring device, comprising a radar sensor which has a transmitter and receiver for transmitting and receiving electromagnetic radiation in the radar and/or microwave range
As a result of the measures indicated in the subclaims, advantageous embodiments and further developments of the invention can be obtained.
Correspondingly, a system according to the invention is characterized in that a radar sensor is provided which has a transmitter and a receiver for sending and receiving electromagnetic radiation in the microwave and/or radar emission range.
Such a radar sensor has a sufficiently high local resolution in order to reliably detect welds of a conventional dimensioning. Since most of the materials are transparent for the radiation of the indicated wave range, a radar sensor is insensitive with respect to exterior environmental influences, such as contaminations, vapors, dust, smoke, light flashes, etc., in the area of the weld. Furthermore, a radar sensor is independent of the light conditions. In particular, it is completely insensitive to the laser radiation frequently used for welding operations.
For analyzing the received signal, reference measurements during a correct welding operation can be taken and the measured data can be collected as reference data in corresponding memory unit. According to the requirements with respect to the quality of the weld, corresponding tolerance criteria can be provided for the different detection parameters, such as the intensity or frequency of the reflection radiation, within which the measured signal corresponds to a correct weld of a desired quality.
In the event of an intolerable deviation of the measured signal from the reference signal, different reactions can take place. Either the welding operation can be controlled or regulated by means of a control value obtained on the basis of the measured data, or the recorded measured data are used for recognizing rejects which can be marked correspondingly. In the event of a faulty weld, it would also be possible to only stop the automatic welding machine and optionally generate a signal until the operator again adjusts the correct welding parameters. Other usage possibilities of the measured signal from the radar sensor exist according to the application.
In an advantageous further development of the invention, the radar sensor and/or the objects to be welded together are arranged such that a relative movement can take place between these two components. As a result, depending on the extent and direction of the relative movement, a Doppler shift of the frequency of the transmitted signals with respect to the received signal takes place. By means of a corresponding analyzing unit for detecting this frequency difference, a frequency spectrum is obtained which is not only dependent on the relative speed of the sensor or of the reflection surface but is also a function of different parameters of the reflection surface, for example, of the shape of the material. This frequency spectrum is therefore also significant for the quality of the weld measured thereby.
In an advantageous further development of the invention, the radar sensor and at least partially the welding tool are fastened on a common holding device. As a result, a space-saving arrangement is obtained, in which case the sensor for controlling the quality of the weld is arranged in the immediate proximity of the weld. According to the application, a possible movement of the welding tool can simultaneously be used as an advance of the radar sensor with respect to the weld to be measured in order to evaluate the Doppler signal, as indicated above.
Also without a controlled relative movement between the radar sensor and the weld, the use of a Doppler radar sensor is advantageous because, as a result of the welding itself, various effects, such as vibrations, etc., are triggered which cause a corresponding Doppler shift.
In a further development of the invention, a waveguide is additionally mounted on the radar sensor. Similar to known optical waveguides, such a waveguide may consist of a solid material or may be constructed as a hollow waveguide. In the case of a corresponding temperature stability, a waveguide can be brought into the immediate proximity of the weld. In this case, the waveguide is preferably constructed as a metallic or ceramic hollow waveguide. For this purpose, a ceramic hollow waveguide is provided with corresponding conducting elements. Surface coatings, the working-in of wire mesh, etc., can, for example, be used.
Such a temperature-stable waveguide can be brought significantly closer into the area of the hot weld than a radar sensor which is temperature-sensitive because of the necessarily existing electronic components.
In a preferred embodiment of the invention, a focussing element is also mounted on the radar sensor and/or on the output of an above-indicated waveguide. Such a focussing element can be mounted, for example, in the form of a so-called horn-shaped emitter or in the form of a radar lens. If the welding operation is to be observed in the direct proximity of the weld, attention should also be paid in this case to the temperature stability of the focussing element.
Because of the currently known materials, a metallic hornshaped emitter will therefore be preferable to a lens. However, it is definitely possible that, on the basis of future materials, lenses will also be available which have the corresponding temperature stability.
The advantages of the focussing are that essentially the actually observable weld is situated in the coverage range of the radiation of the radar sensor and this weld therefore exercises the predominant influence on the response signal. Changes in the course of the weld are therefore indicated correspondingly more clearly in the response signal of the radar sensor.
The transmission frequency of the radar sensor is preferably selected to be narrow-band. Definitely, several frequencies can be used with a corresponding spacing. As a result, while utilizing a relative movement between the radar sensor and the objects to be welded together, a Doppler spectrum will have a higher informative value because there can be no superimposing of response signals from different transmission frequencies.
Furthermore, it is recommended to adapt the wavelength of the transmitted signal to the width of the weld to be measured. In this case, the wavelength should be in the range of the weld width or below in order to ensure a sufficient local resolution.
By means of the invention, all possible weld formations can be monitored, particularly also weld points and weld seams.
By means of a radar sensor, the weld and its environment is monitored, for example, with respect to movement and vibrations, which can be detected particularly by means of the Doppler radar. Simultaneously, by means of this operation, the welding system itself, for example, during electro-welding, the electrode geometry and the electrode movement, can be monitored. This permits an additional process control in order to recognize damage and wear of the welding system, particularly of electrodes, early.
Simultaneously, in addition to controlling the quality of the weld, the monitoring of the exact position of the parts to be welded together is permitted. Thus, for example, during stud welding, the stud position and the stud geometry can be simultaneously monitored. Also this examining method is based, among other things, on an analysis of surface vibrations of the parts to be welded together, for example, of a sheet metal part during the welding operation.
When a welding operation is considered in detail, the following individual steps may, for example, be obtained:
1. The parts to be welded together, such as metal sheet or other metallic parts, are positioned between, for example, rod-shaped electrodes, and the welding operation is triggered.
2. The two electrodes move toward one another and contact the parts at a defined maximal pressure.
3. The energizing of the electrodes takes place shortly after the contact and is controlled by way of the electrode position.
4. As a result of the high current density and the corresponding material resistance, the material becomes very hot locally and deforms; that is, it becomes plastic and possibly even liquid.
5. The electrodes are guided to follow, the penetration depth of the electrodes normally being used as the control parameter for the welding operation. In addition, this penetration depth, as a rule, is mechanically limited.
6. Subsequently, the current supply is switched off and the electrodes, which move apart, release the welded-together part.
During these operations, that is, particularly during the contacting and the releasing of the welded-together part or of the parts to be welded together by the electrodes, vibrations are caused or influenced predominantly when welding sheet metal.
Thus, the material vibrations, for example, typically vary with respect to time during the buildup of the contact pressure forces in the surroundings of the electrode contacting. During the plasticizing or the liquefying of the material, this vibration is abruptly damped and is therefore also clearly influenced. During the moving-apart of the electrodes and the connected hardening of the weld, vibrations occur again which clearly differ from the preceding signals of the unwelded parts.
These vibration processes of the welded parts or metal sheets in the vicinity of the weld can be tracked without contact by means of the radar sensor and can be analyzed by means of the corresponding analyzing unit.
The variation of the vibrations at the welded parts or metal sheets takes place as in other welding processes, for example, during arc welding, which is frequently used for welding studs onto sheet metal parts.
Another embodiment of the invention consists of causing the area of the weld to vibrate as a result of independent excitation before, during or after the welding-together and of observing these vibrations by means of one or several radar sensors. If parts are not welded together or are not optimally welded together, a vibration behavior occurs in this case which differs from a correct welding-together and which is reflected in the corresponding sensor signal.
This independent excitation can be used, for example, during point welding, in which the entire welding operation takes place very rapidly and the vibrations caused thereby decay relatively fast. As a result of the independent excitation, a weld can still be checked also at a later point in time after the decaying of the vibrations triggered during the welding.
Beyond the observation of the vibration behavior, by means of a radar sensor, the change of the reflection surface during the welding operation can also be observed directly. Thus, during the transition of the rigid partial areas to be welded to one another into the plastic phase or molten phase, a corresponding significant change in the radar signal takes place in the area of the weld. The surface of the weld, in turn, changes during the cooling and solidifying which itself is again indicated in the sensor signal of the radar sensor.
An embodiment of the invention is illustrated in the drawing and will be explained in detail in the following by means of the figure.